1. Field of the Invention
The present invention relates to a DSRC system that is a short range radio continuous communication system considered as an important technique for supporting Intelligent Transport Systems (ITS).
2. Description of the Related Art
The short range radio communication (hereafter, referred to as DSRC (Dedicated Short Range Communication)) system will be described below.
An example of a communication frame used in the DSRC is shown in FIG. 8A and FIG. 8B. In the case of the full-duplex communication system, there are two kinds of periods: 3.91 ms and 2.34 ms as the period of the frame, and either frame is applied according to the size of the communication range. The transmitting and receiving slot allocation information in each frame is stored in a FCMS (Frame Control Message Slot) that is the head slot of the frame, and on the basis of this information, a roadside device and an on-vehicle device perform the transmitting and receiving of data by using a MDS (Message Data Slot).
The concrete operation of the DSRC will be described by taking the ETC (Electronic Toll Collection) as an example. In the case of the ETC, the number of roadside devices to be provided is different depending on the tollgate. First of all, an example of the exit tollgate is shown.
An example of the configuration of the roadside device at the exit tollgate is shown in FIG. 4. The roadside device comprises: an application processing section 214 which performs an application processing of the ETC that is a non-stop toll collection system of a toll calculation or the like; a roadside antenna 201 which performs the radio communication by using a frequency of 5.8 GHz with the on-vehicle device; a high frequency section 211 which down-converts a signal of 5.8 GHz from the roadside antenna 201 and makes it a base band signal, and which up-converts a base band signal to 5.8 GHz on the contrary; a base band section 212 which performs a generation of a communication frame, a generation of transmitting data, and an error check of receiving data or the like; and a DSRC control section 213 which performs a DSRC protocol processing on the basis of signals from the application processing section 214 and the base band section 212.
Next, the configuration of the on-vehicle device is shown in FIG. 7. The on-vehicle device comprises: an application processing section 405 which performs a toll accounting notice to a driver and a toll accounting information writing to an IC card or the like; an on-vehicle antenna 401 which performs the radio communication by using a frequency of 5.8 GHz with the roadside antenna; a high frequency section 402 which down-converts a signal of 5.8 GHz from the on-vehicle antenna 401 and makes it a base band signal, and which up-converts a base band signal to 5.8 GHz on the contrary; a base band section 403 which searches or monitors the FCMS from the roadside antenna and performs a synchronization of the communication frame, and which performs a generation of transmitting data and an error check of receiving data or the like; and a DSRC control section 404 which performs the DSRC protocol processing on the basis of signals from the application processing section 405 and the base band section 403.
An example of the exit tollgate is shown in FIG. 3A and FIG. 3B. At the exit tollgate, one roadside device is provided in the direction of movement of the vehicle. When started, the on-vehicle device 203 searches the FCMS from the roadside antenna 201 at all times. When the on-vehicle device 203 enters a communication range 202 of the roadside antenna 201, it starts to receive the FCMS from the roadside antenna 201, and if it continuously and normally receives the FCMS, it issues a request of link establishment to the roadside antenna 201, and performs a accounting process of an expressway toll or the like.
On the other hand, at the entrance tollgate, for the processing of the detection of the information on the type of a vehicle or the like, two roadside devices are provided in the direction of movement of the vehicle. An example of the entrance tollgate is shown in FIG. 5A and FIG. 5B. By the radio communication between the on-vehicle devices 303, 306 and the first roadside antenna 301 that is the front roadside antenna in the opposite direction of movement of the vehicle and the second roadside antenna 304 that is the rear antenna, the receiving and transmitting of the entrance information and the information on the type of a vehicle are performed. The above described two antennas (the first roadside antenna 301 and the second roadside antenna 304) are provided adjacently, and therefore, there is a possibility of causing a communication obstacle because of the radio wave interference. Therefore, in the case where the configuration shown in FIG. 5A is adopted, as shown in FIG. 5B, the communication frames are alternately operated. This action where communication frames are alternately operated by the front and rear antennas is called the time sharing operation.
An example of the configuration of two roadside devices at the entrance tollgate is shown in FIG. 6. Similarly to the example at the exit tollgate, each of two roadside devices comprises: antenna sections 301 and 304; high frequency sections 321 and 331; base band sections 322 and 332; an application processing section 324; and DSRC control sections 323 and 333. The application processing section 324 is connected to both the DSRC control sections 323 and 333 to perform a series of processing as an application.
Furthermore, the DSRC control sections 323 and 333 are synchronized by receiving and transmitting a synchronizing signal 341, and the operation of the communication frames is performed so that at the timing when one roadside device communicates, the other stops.
Next, the concrete processing at the entrance tollgate will be described by referring to FIGS. 5A, 5B and FIG. 6. FIG. 5A is a drawing showing the communication range, and FIG. 5B is a drawing showing the communication frame. At the timing when the first roadside antenna 301 operates, the first roadside antenna 301 and the on-vehicle device 303 in the communication range 302 of the first roadside antenna 301 perform the communication by the communication frame 311 of the first roadside antenna 301, and during that period, the second roadside antenna 304 and the on-vehicle device 306 in the communication range 305 of the second roadside antenna 304 do not perform the communication (timing of the stopping frame 314 of the second roadside antenna 304).
Similarly, at the timing when the second roadside antenna 304 and the on-vehicle device 306 perform the communication by the communication frame 313, the first roadside antenna 301 and the on-vehicle device 303 are in the timing of the stopping frame 312, and do not perform the communication. Thus, in the case of two adjacent antennas, the interference of the radio wave is avoided by using the time sharing operation where at the timing when one performs the communication, the other does not perform the communication at all.
Next, the operation of the on-vehicle device will be described in detail. When started, the on-vehicle device enters the FCMS search mode to attempt the receiving of the FCMS from the roadside antenna.
When the on-vehicle device succeeds in the receiving of the FCMS, it attempts the receiving of the FCMS again, and if it continuously succeeds in the receiving of the FCMS, it moves to the FCMS monitoring mode. At the FCMS monitoring mode, the timing of receiving the FCMS is fixed, and whether the FCMS can normally be received at the fixed timing or not is monitored.
In the FCMS monitoring mode, when the on-vehicle device continuously fails in the normal receiving of the FCMS, it releases the fixing of the FCMS receiving timing, and returns again to the mode of searching the FCMS.
When taking the above described entrance tollgate as an example, the above described action is as follows. In FIG. 5A, the on-vehicle device 303 enters the communication range 302 of the first roadside antenna 301 while operating in the FCMS search mode. The boundary area of the communication range 302 is an area where the radio wave is unstable, and hence the on-vehicle device 303 repeats the action of succeeding in or failing in the receiving of the FCMS, and becomes in the state where it enters the FCMS search mode or enters the FCMS monitoring mode.
When the on-vehicle device 303 advances to enter the communication range 302, the area becomes an area where the radio wave is stable, and it becomes possible to stably and normally receive the FCMS, and therefore, the mode becomes the FCMS monitoring mode. After that, furthermore advancing to the area to come out of the communication range 302, the area becomes an area where the radio wave is unstable again, and similarly to the time of entering, the FCMS search mode and the FCMS monitoring mode are repeated. Finally, the FCMS from the first roadside antenna 301 cannot arrive at all, and the on-vehicle device 303 returns again to the FCMS search mode.
In the case of a system where roadside antennas are arranged in a line in the direction of movement of the vehicle on an expressway or the like, and the DSRC is continuously performed during the period when a vehicle runs on the expressway, as described above, the on-vehicle device performs the communication with one antenna, and continues the communication as long as it can normally perform the communication with that antenna. If it becomes impossible to normally perform the communication with that antenna, the on-vehicle device once resets the state, and searches a roadside antenna with which it can normally perform the communication. Therefore, at the boundary area where the on-vehicle device enters the communication range of the next antenna from the communication range of a certain antenna, the communication is inevitably interrupted.
Furthermore, the method of providing the roadside antennas is also difficult. If the roadside antennas are provided so that the communication ranges are overlapped onto each other, an interference of the radio wave is caused at the overlapped part, and it becomes impossible to perform the normal communication, and the communication is interrupted. On the contrary, if the roadside antennas are provided with a distance to avoid the interference, it also becomes impossible to normally perform the communication at the part where a space is made, and the communication is interrupted.